1. Field of the Invention
The invention relates to a field emission type emitter and a method of manufacturing thereof which can be suitably applied to a flat pannel display such as a flat CRT.
2. Description of the Prior Art
Hitherto, as a field emission type emitter of the size of the micron order, a emitter called a Spindt type has been known. Its manufacturing method is as follows.
As shown in FIG. 1A, after a silicon dioxide (SiO.sub.2) film 2 was first formed onto a conductive silicon (Si) substrate 1 by a thermal oxidation method, a CVD method or a sputtering method, a molybdenum (Mo) film 3 is formed onto the SiO.sub.2 film 2 by a sputtering method or an electron beam evaporation deposition method as a material to form a gate electrode. A thickness of the SiO.sub.2 film 2 is about 1 to 1.5 .mu.m. A thickness of the Mo film 3 is thousands .ANG., for example. After that, a resist pattern 4 having a shape corresponding to a gate electrode to be formed is formed onto the Mo film 3 by a lithography.
Then, the resist pattern 4 is used as a mask and the Mo film 3 is etched by a wet etching method or a dry etching method, thereby forming a gate electrode 5 as shown in FIG. 1B. The gate electrode 5 has an opening 5a having, for example, a circular shape having a diameter of about 1 .mu.m.
Subsequently, the resist pattern 4 and the gate electrode 5 are used as masks and the SiO.sub.2 film 2 is etched by a wet etching method, thereby forming a cavity 2a as shown in FIG. 1C.
After the resist pattern 4 was removed, an oblique evaporation deposition is executed by an electron beam evaporation deposition method in a direction with a predetermined inclination angle to the substrate surface, thereby forming a peeling-off layer 6 made of, e.g., aluminum (Al) on the gate electrode 5 as shown in FIG. 1D. The oblique evaporation deposition is executed while the Si substrate 1 is rotated around its center.
Then, Mo is evaporation deposited by an electron beam evaporation deposition method as a material to form a cathode in the direction perpendicular to the substrate surface. Due to this, as shown in FIG. 1E, a cathode 7 is formed on the Si substrate 1 in the cavity 2a. Reference numeral 8 denotes a Mo film formed on the peeling-off layer 6 at the time of the evaporation deposition. A thickness of the Mo film 8 is about 1 to 2 .mu.m.
After that, the peeling-off layer 6 is removed by a lift-off method together with the Mo film 8 formed thereon, thereby completing a target field emission type emitter as shown in FIG. 1F.
Since it is necessary to execute the electron emission from the cathode 7 in the vacuum of about 10.sup.-6 Torr or less, the above field emission type emitter is actually sealed in the vacuum by opposite plates and other members (not shown).
The above conventional field emission type emitter shown in FIG. 1F has the following many drawbacks. That is, since the above refractory metal such as Mo which is used as a material of the gate electrode 5 is likely to be oxidized, the gate electrode 5 is easily oxidized in the manufacturing process and an electric conductivity decreases. Thus, the electron emission from the cathode 7 cannot be stably performed. There is also a case where a deformation of the gate electrode 5 occurs due to the oxidation. Further, since an internal residual stress due to a film formation of the refractory metal such as Mo or the like is large, a deformation of the gate electrode 5 easily occurs. Consequently, the gate electrode 5 is easily peeled off from the SiO.sub.2 film 2.
Further, in the manufacturing method of the above conventional field emission type emitter, to actually execute the lift-off of the Mo film 8, it is necessary that an etchant solution for lift-off reaches the peeling-off layer 6 under the Mo film 8. However, as shown in FIG. 1E, the Mo film 8 covers the substrate surface almost completely, so that a thin portion of the Mo film 8 just over the cathode 7 is the only place where the etchant solution for lift-off can enter below the Mo film 8. Therefore, the etchant solution for lift-off is hard to reach the peeling-off layer 6, so that it is difficult to actually execute the lift-off.
This problem takes a remarkable effect, especially in the case of forming a large area field emission type emitter array. That is, in the field emission type emitter array, the pitch of the cathode 7 is set to, e.g., about 10 .mu.m, while the diameter of the opening 5a of the gate electrode 5 which is formed just over the cathode 7 is set to about 1 .mu.m and is very small as compared with the pitch of the cathode 7. In this case, there is no place where the etchant solution for lift-off can enter below the Mo film 8.
As a result, the lift-off could not be executed partly or the thin Mo film 8 was left on the peeling-off layer 6, and the lift-off could not be executed completely. In addition, even if the lift-off could be executed completely, the lift-off took a fairly long time and the productivity was low.
Further, since the conventional field emission type emitter shown in FIG. 1F mentioned above has an overhanging structure in which the gate electrode 5 is projected to the inside of the cavity 2a in parallel with the substrate surface, there are problems such that the gate electrode 5 is weak in terms of structure and a peel-off or the like from the SiO.sub.2 film 2 is likely to occur.
On the other hand, a field emission type emitter of a structure as shown in FIG. 2 has also been known. As shown in FIG. 2, in the field emission type emitter, the side walls of a cavity 12a formed in an SiO.sub.2 film 12 are perpendicular to the substrate surface. Such a cavity 12a is formed by a reactive ion etching (RIE) method. Reference numerals 11, 13, and 14 denote a Si substrate, a cathode and a gate electrode, respectively.
The conventional field emission type emitter shown in FIG. 2 has a structure such that the whole gate electrode 14 is supported by the SiO.sub.2 film 12, so that the gate electrode 14 is strong in terms of structure. In this case, however, there are the following problems. That is, in the case of actually forming the cavity 12a by an RIE method, it is not always easy to control the shape of the bottom portion because a diameter of cavity 12a is small. Therefore, there is a case where the side walls of the cavity 12a are not always perpendicular to the substrate surface and a diameter of bottom portion is small. In such a case, there are fears such that a defective shape of the cathode 13 which is formed in the cavity 12a occurs and a defective insulation between the cathode 13 and the gate electrode 14 occurs.